Synctus (my startup: http://www.synctus.com) does something similar. It is an always-on hardware appliance for each office that Just Works - including NAT traversal. It presents an SMB/CIFS share that is the same regardless of which office you are at. Drop me a line if you think it could be useful for you.
Synctus is different from Dropbox and presumably SparkleShare because it focuses on emulating a single office SMB/CIFS share exactly, including locking so that synchronisation conflicts never happen - and this means that it needs each appliance to be usually-on (so road warriors still need to VPN in to access a particular share or use Dropbox or similar).
Neat product. This won't work for us since I'm trying to get away from CIFS (it's the transition that's hard), but I can imagine many situations where this would be very useful.
Sure beats fighting with windows replication, and has a lot of benefits over rsyncing.
Synctus is different from Dropbox and presumably SparkleShare because it focuses on emulating a single office SMB/CIFS share exactly, including locking so that synchronisation conflicts never happen - and this means that it needs each appliance to be usually-on (so road warriors still need to VPN in to access a particular share or use Dropbox or similar).